1. Field of the Invention
The invention generally relates to video decoders and more specifically to systems and techniques for correcting the time base of a sampled composite video signal wherein the sample clock and video source clock are asynchronous.
2. Description of Related Art
There is a large surge in the use of digital video devices today. Examples include: digital televisions, LCD (Liquid Crystal Display) TVs (televisions) and monitors, DVD (Digital Versitile Disc) recorders, personal video recorders, PC (Personal Computer) video cards, video capture and streaming applications, and video conferencing. In many cases, these units need to receive an analog video signal, which may be one of the composite signals, such as NTSC (National Television Standards Committee), PAL (Phase Alternating Line), SECAM (Sequential Couleur Avec Mémoire), S-video, component video, or RGB (Red, Green, Blue). It is then desirable to produce the proper digital output, such as eight or ten bit ITU-R (International Telecommunication Union-Radio-Communication) BT (Broadcasting Service-television) 656. It is preferred that all the video decoding be done in a single chip for all of these formats. The decoder not only has to handle composite signals, which means it must be able to determine the chroma and luma values, but it also must handle vertical blanking interval (VBI) data and handle VCR (video cassette recorder) signals, which may be unstable signals.
Although a number of such systems have been developed, it is always desirable to improve the output and capabilities of the particular video decoder. For example, it is desirable to sample a composite video signal with a sample clock that is asynchronous with the video source clock. This provides for increased noise immunity and thus higher video quality. However, these gains can only be realized if the time base of the sampled composite video signal is converted from the sample clock back to the video source clock so that brightness and color information contained in the video signal can be more accurately recovered.
Therefore, systems and methods for converting signals from a sample clock domain to a source clock domain are needed. It would be further advantageous to recover the source clock entirely in the digital domain such that there is minimal noise injected into the video decoding process, whether as a result of clock noise or other means.